The present invention relates to testing of networks, and more particularly to a method and apparatus for non-intrusive measurement of end-to-end properties of data transmission through a network path.
A network provides uni-directional virtual or physical path or channel for data transmission. A network flow is a session of data transmission through a network path. In a network flow data units flow from an ingress to an egress of the network path. Example networks are Internet Protocol (IP) networks, Asynchronous Transmission Mode (ATM) networks and circuit-switched, such as telephony, networks. Example data units are IP packets, ATM cells and data bytes (octets). For simplicity the following is described in the environment of IP networks, so the terms IP networks, IP virtual path, IP packet and packet flow are used.
The end-to-end properties of a packet flow may be measured by several metrics, such as throughput, end-to-end latency, packet loss, etc. There are two approaches to measuring the metrics. One is active and the other is passive.
In an active approach packets for monitoring purposes, with special identities and time-stamps, are injected into the ingress of a packet flow. At the egress these special monitoring packets are identified, removed from the flow and time-stamped. Metrics, such as end-to-end latency and packet loss may be measured by analyzing the received monitoring packets. An advantage of the active approach is that it is straightforward and simple, and may be used either online or off-line. The main disadvantage of the active approach when used online is that, no matter how small the injected monitoring packet flow is, these monitoring packets always interfere with the operational packet flow. Also, since the network condition is dynamic, off-line measurement might not reflect the actual properties of the packet flow.
In a passive approach no monitoring packets are injected into the packet flow being monitored. Instead at both the ingress and egress of the packet flow the operational data packets are intercepted without interference to the packet flow itself, and measurement data is generated for each data packet. The measurement data from the two points are correlated, and end-to-end property metrics are derived. In contrast to the active approach the passive approach is non-intrusive. It does not interfere with the operational packet flow in any manner. The cost of this benefit is that the passive approach is much more complex and harder to implement. Due to the reliance on the operational packet flow, the passive approach does not assess the end-to-end properties of a network path before its actual operation.
A passive approach for ATM network monitoring systems is the WAND project by the University of Waitato in New Zealand which is disclosed at http://atm.cs.waikato.ac.nz/wand. WAND focuses on delay measurement for ATM networks, specifically uses a CRC for data correlation (matching of ATM cells), and matches one cell from the ingress to a list of cells from the egress.
What is desired is a non-intrusive method of monitoring end-to-end properties of network flows in an efficient way.
Accordingly the present invention provides a method and apparatus for non-intrusive measurement of end-to-end properties of network flows using a passive approach. An ingress monitor non-intrusively intercepts data units as they enter a network path through a network. Likewise an egress monitor non-intrusively intercepts the same data units as they leave the same network path. Each monitor generates a time stamp for each intercepted data unit using a common clock and derives a unique signature for each data unit such that the same data unit has the same signature at the entry as at the exit. Additionally each monitor counts the number of packets received from the network flow at the ingress and egress respectively. The signature, time stamp and packet counter value form an entry which is retained in an entry queue in each monitor. A data correlator coupled to an out-of-band network to which the monitors also are coupled periodically pulls a list of entries from each monitor and correlates the lists by matching signatures. The time stamps for the entries from the respective lists having the same signature are processed to obtain end-to-end latency of the network path, and the difference in counts over a given time period provides end-to-end packet loss measurements.
The objects, advantages and other novel features of the present invention are described in the following detailed description when read in conjunction with the appended claims and attached drawing.